A wireless channel in a mobile communication system has very low reliability due to effects such as fading. In some cases, data transmitted via such a wireless channel may not be decoded successfully at a receiving end. In this case, the receiving end should inform a transmitting end whether the decoding is successful so that transmitted data can be re-transmitted when necessary. A channel for informing whether the reception is successful is called an ACK/NACK channel. A reception success signal informing when the reception is successful at a receiving end is an ACK signal and a reception failure signal informing when reception has failed is called a NACK signal. As such, ACK/NACK information is generally 1 bit information.
In general, when the ACK/NACK signal is transmitted separately, the ACK/NACK signal is transmitted by performing Binary Phase Shift Keying (BPSK) modulation.
When it is assumed that transmission power isP, 
a signal of 1 is transmitted for the ACK signal, and a signal of −1 is transmitted for the NACK signal, a signal of√{square root over (P)}
is transmitted for the ACK signal and a signal of−√{square root over (P)}
is transmitted for the NACK signal from a transmitting end. In addition, when it is assumed that a signal sent from a transmitting end iss and a wireless channel between a transmitter and a receiver ish, a reception signal of a receiving end{tilde over (y)}is as given by Equation 1.{tilde over (y)}=hs+ñ  [Equation 1]
wherein
ñ
is Additive White Gaussian Noise (AWGN).
When the real number part of the reception signal{tilde over (y)}
is a positive number, it is interpreted as an ACK signal and when the real number part of the reception signal{tilde over (y)}
is a negative number, it is interpreted as a NACK signal.
Unlike such a conventional method, only when decoding is successful at a receiving end, an ACK signal may be transmitted, and when decoding has failed, no signal may be transmitted.